Different devices for detecting the level of liquid metal in a casting apparatus are known, which use different reading principles.
For example, devices with radioactive isotopes are known, which comprise an emitter which emits radiation in the direction of the crystallizer containing the liquid steel, and a receiver positioned opposite the emitter on the opposite side of the crystallizer, which receives the radiations. In relation to the intensity of radiations received it is possible to determine the level of liquid metal present in the crystallizer, interposed between the emitter and receiver.
These devices have the disadvantage, however, that they do not distinguish the actual level of the meniscus of the liquid metal with respect to an overlying covering layer of powders which is normally deposited as a protection for the metal bath and as lubrication.
Devices of the thermic type are also known, which detect the development of the temperature profile of the crystallizer due to the presence of the liquid steel, but these have the disadvantage that they are subject to errors due to perturbations of the heat flow between the liquid steel and the crystallizer, generally due to the instability of the lubrication function performed by the protection powder.
Among the devices to detect the level of metal liquid of the electromagnetic type, for example those shown in EP-A1-312,799 and U.S. Pat. No. 3,670,801, a device is known which can be applied to crystallizers both of the plate and tubular type which exploit the variable magnetic field generated by an excitation coil localized in proximity to a perimeter point on the upper end of the crystallizer, in order to induce induced currents in the liquid steel present in the ingot mold.
By striking the liquid steel with a variable magnetic field, induced currents are known to be generated therein.
The induced currents in the liquid steel, in their turn, generate an induced reaction magnetic field which is detected by suitable reception coils of the device, localized adjacent to the excitation coil.
The reaction magnetic field is amplified and processed by a control unit to determine, depending on its intensity, the level of liquid metal in the crystallizer.
In particular the device is disposed above the crystallizer and directly in proximity to its wall, so that one of its surfaces sensitive to emission/reception, in correspondence to which the excitation and reception coils are disposed, is substantially coplanar with the wall of the crystallizer.
More specifically the device has a rather compact shape, its sensitive surface is substantially conformed with the same shape as the wall of the crystallizer and covers only a determinate limited sector of the latter.
One disadvantage of these known devices is that, above all in the case of large cast products, the detection of the level of liquid metal is concentrated to a limited detection zone, dictated by the size of the device and by the intensity of the excitation currents of the excitation coils, which is, however, limited by technical parameters.
Moreover, when it is necessary to detect the level of molten metal in large ingot molds it is necessary to pre-dispose several devices, equidistant to each other, along the periphery of the ingot mold, thus entailing a considerable complexity in application, and also in processing the signals detected.
The magnetic field generated by the excitation coils of known devices is subject to deviation effects of the field lines, substantially due to the walls of the crystallizer which are normally made of copper.
The deviation effects are even more accentuated in proximity to the lateral edges of the sensitive surface which is practically in contact with or in any case coplanar to the crystallizer walls.
One purpose of the present invention is to achieve a device which allows to detect with precision the average level of the meniscus of the liquid metal also for large cast products.
Another purpose of the present invention is to achieve a device to detect the level of liquid metal in a casting apparatus which allows to detect with precision the actual level of liquid metal, limiting or in any case controlling the disturbance effects due to the copper walls of the crystallizer or to other causes.
It is also a purpose of the present invention to achieve a detection device which is reliable, economical and simple to make and install on the ingot mold body.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.